Power supply circuit for continuous-wave magnetron

ABSTRACT

A power supply circuit for a continuous wave magnetron provides selectively adjustable microwave power levels that are stable at each level. The unregulated direct voltage supplied to the magnetron cathode is combined with an additional voltage derived from a common alternating current source to provide a total direct current which passes through the electromagnet coil of the magnetron. Variations in the power source are compensated by the variations of the added current in the coil to provide stable magnetron power at each level. The circuit providing the added current to the coil includes a simple rectifier, filter, zener diode and series resistor arrangement with a switch for selecting different diode, resistor and voltage combinations.

POWER SUPPLY CIRCUIT FOR CONTINUOUS-WAVE MAGNE'IRON Inventor: Jean Paul Maillot, Berthevin,

France Assignee: International Standard Electric Corporation, New York, N.Y.

Filed: Sept. 10, 1971 Appl. No.: 179934 Foreign Application Priority Data Sept. 18,1970 France. 7033903 US. Cl. ..328/230, 321/43, 315/101, l

328/262, SIS/39.51, 331/87 Int. Cl ..H01j29/72 3 Field of Search "328/230, 262; 331/86, 87;

References Cited UNITED STATES PATENTS 4/1961 Schall ..328/262 Primary Examiner-John Kominski Attorney-C'Cornell Remsen, Jr.

[571 ABSTRACT A power supply circuit for a continuous wave magnetron provides selectively adjustable microwave power levels that are stable at each level. The unregulated direct voltage supplied to the magnetron cathode is combined with an additional voltage derived from a common alternating current source to provide a total direct current which passes through the electromagnet coil of the magnetron. Variations in the power source are compensated by the variations of the added current in the coil to provide stable magnetron power at each level. The circuit providing the added current to the coil includes a simple rectifier, filter, zener' diode and series resistor arrangement with a switch for selecting different diode, resistor and voltage combinations.

7Claims,5DrawingFigures PA'TENTEDnnv 14 1912 SHEET 3 [IF 3 Inventor JEAN PAUL MA/LLO T u A NQ av vsfm I v POWER SUPPLY CIRCUIT FOR CONTINUOUS- WAVE MAGNETRON BACKGROUND OF THE INVENTION are particularly designed for use in industrial applications where high microwave power must be permanently available. In most cases, such applications relate to insulator materials whichare more or less wet and which are dried by heating, such heating resultingfrom dielectric losse within insulator materials.

Continuous-wave magnetron operation is in noway different than other pulsedmagnetron operations such as, for example, those used in radar. However, needs for. obtaining high continuous power under economical conditions require a specific technology and, particularly, a specific mode for generating high voltage and induction fields which control the magnetron current and consequently the microwave power.

It is often very important for usersof heating magnetrons to obtain microwave power which is adjustable and stable for each adjustment.

Some presently known power supply devices permit varying the available microwave power in discrete stepped levels, but none provides satisfactory results as far as simplicity, device reliability or power stability. at each level are concerned.

in order to better understand continuous-wave magnetron operation and the difficulties usually foundin known power supply devices, it is noted that electrical magnetron characteristics within the useful range may be represented by a linear law of the kind:

I=lr(V-bB) 1) in this formula: I is the magnetron current intensity, V is the high voltage,

B is the magnetic induction field,

r is the internal dynamic resistancewhich is. of some tens of ohms, and b is very large, which means that a small variation of B causes a large variation on I, V being constant. By way of comparison, B may be said to be acting, like the grid voltage in a high slope triode.

For predetermined values of the high voltage V the applied electric power is:

P VI (2) The microwave power supplied to theload is equal.

P, k VI where k is the magnetron efficiency. Experience shows that, if the load is sufficiently well matched, k is substantially a constant over the whole useful range of tube characteristics, k being usually about 70 to 80 percent.

itis conceivable that a continuous orstepped variation of V and/or B will permit adjustment of P, and consequently P,,,, in a predetermined manner.

In a first type of application, the induction B may be provided by a permanent magnet. In that case, B is steady and P,,, canvary only if the high voltage V is varied.

The supplied power, for a predetermined adjustment of V, is very sensitive to instabilities of V.

A simple calculation using the formulas (1) and (2) leads to the following relation expressing the relative variation of power APm/Pm: I

APmlPm= (l V/rl) AV/V For a usual magnetron having the following values;

I l A r= ohms it is found that; I

- APm/Pm 50 AV/ V Therefore, in this case, the power regulation requires very good stabilization of the high voltage V.

In practice, V is generated by an ac. source whose current is rectified by well known means. The need for simultaneously being able to adjust Pm by discrete levels and for good regulation of each level generally requires complex and costly power supply devices which often use a saturated-current transformer. This precludes varying V and Pm, in a continuous manner.

In a second type of application, for generating B, an electromagnet is used which has a. coil traversed by a current I In a first approximation, B may be assumed to be proportional to the magnetic field created by the coil, or proportional to the current I The formula (1) may be then written:

I=1/r(Vr I V/r-aI (3) Taking into account the rapid variations of I with I r /r a is a coefficient substantially higher than 1.

Pm can be adjusted very simply by continuous or stepped variations of the current 1 V being steady). 1 is generated by a source of relatively low voltage V, obtained after having rectified the source of alternating current. If, for a predetermined adjustment, V and V, are subject to the same relative variations, equal to those of anac. source of voltage U, a simple calculation shows that:

' APm/Pm 2 AU; U

The regulation of Pm is therefore easier to obtain than in the case of use of a permanent magnet and requires substantially less accurate stabilization of the sources. The main drawback of this type of power supply is in the very high sensitivity APm/AI wherein a low variation of I that is not compensated by an associated variation of V may cause a large variation of Pm.

In certain embodiments of this second type, a part P of the magnetron current I is used as current 1,, The properly sized coil is electrically connected between the tube anode and the ground. An adjustable resistor is mounted in parallel across the coil and permits variation of p and consequently of the power Pm within a particularrange.

The formula (3) shows that the power Pm is equal to:

If V is constant, the power Pm is regulated by variation of p. The maximum power corresponds to the minimum value of p and the minimum power to the value p l. in this last case, a shunt resistor is suppressed.

The device is simple, but it does not provide sufficient variations of Pm. Moreover, as previously: APm/Pm=2 AV/V=2 AU/U The current 1,, will usually be produced by a source of rectified voltage V, supplying a resistor R connected in series with the coil of resistance R The calculation then shows that l is given by the formula:

and the generated microwave power by Proper selection of the values of V and R, by varying V with R and V being constant, or R with V and V,, being constant, or V, and R simultaneously with V being constant, results in a continuous variation or in discrete levels, of Pm from a maximum value to a very low value.

V and V are supplied by an a.c. source after rectification. If, for a predetermined adjustment, they are subject to the same relative variations, still equal to those of an a.c. source of volt-age U, a calculation using the formula (6), shows that:

' APm/Pm 2 AU/ U This last type of power supply is more advantageous than the preceding ones because the sensitivity APm/A- Va may be low.

However, good regulation of Pm requires sufficient stabilization of the alternating current source from which V and V,, are derived.

SUMMARY OF THE INVENTION A purpose of the present invention is to provide an additional source of current that is added to the magnetron current in the induction coil, such that for each setting of the voltage of the additional source there is a corresponding value of delivered microwave power which is substantially independent of the instabilities of the common a.c. power source or of the magnetron power supply high voltage and of those of the additional source voltage.

According to the invention, the circuits associated with the additional source are designed so that, for each adjustment of the voltage thereof, or of the associated circuits and of the microwave power Pm, the effects of the instabilities of the high power supply voltage V are compensated by those of the instabilities of the additional source to maintain Pm stable.

According to a feature of the invention, the additional source of voltage V, is replaced by a source of voltage V, having an output connected to a non-linear circuit comprising in series an element of very stable counter electromotive force (c.e.m.f.) E and very low internal resistance, a resistor R and the induction coil. V is given by the relation V, V, E,,.

According to another feature of the invention, the magnetron operating at fixed high voltage V, the c.e.m.f. E the resistor Rand the magnetron current intensity I corresponding to a predetermined adjustment of the delivered microwave power are related by the formula:

v E, 2R1.

According to another feature of the invention, the

additional source voltage V, is equal to V, E, E,,/2 (l I E being the voltage across the induction coil terminals and I; being the intensity of the current passing through the said coil.

According to an embodiment of the invention, to obtain q regularly spaced discrete levels of microwave power, the non-linear circuit includes q identical Zener diodes and a series resistor R. Decreasing microwave power levels are obtained by successively short-circuiting one, two, three, (q-l) Zener diodes and by simultaneously lowering the voltage V, of the additional source by e,,/2, e, being the equivalent Zener diode counter electromotive force.

According to another embodiment of the invention, to obtain q discrete microwave power levels, the nonlinear circuit includes a single Zener diode and q resistors in series. Increasing microwave power levels are obtained by successively short-circuiting one, two, three, (q-l) resistors while each corresponding voltage V,,' is made to vary linearly with the value of the resistance of the circuit. In addition, in order to obtain q discrete microwave power levels, the additional source voltage V,,' is constant and for each microwave power level a coupling circuit comprising a Zener diode and a resistor R is used.

According to a further embodiment of the invention, wherein the microwave power is continuously varied, the q resistors are replaced by a rheostat and the a.c. source which generates the voltage V comprises a variable autotransformer with a sliding contact, the movement of the sliding contacts of the rheostat and of the autotransformer being properly coupled. The main advantage of the power supplies of the present invention is the excellent regulation of the microwave power delivered by using a non-stabilized high voltage and an additional voltage source, and consequently a reduced cost from that of power supplies used in the prior art. Only two efiicient low pass filters are required.

The various purposes and features of the present invention will appear more clearly from the following description of embodiments, taken in conjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the family of characteristics I f V, I of a continuous-wave magnetron,

FIG. 2 shows the family of characteristics I =flI,,, V), 1,, being the intensity of the current supplied by an additional source, and

FIGS. 3, 4 and 5 show three different embodiments of power supply circuits according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the characteristic curves of the magnetron current I with the high voltage V for various values of the current I, passing through the magnetron electromagnetic coil. With V constant, as shown by the dashed line, the current I decreases rapidly as I, increases slowly. In other words, the applied power P VLvaries from a maximum value down to a quasi-null value with a relative increase of 1,, which practically is of about 1 to 2.

As previously mentioned, the current I is given by the formula (3): v

I V/r a 1 wherein r is the internal dynamic resistor of the magnetron (of about a few tens of ohms) and a is a coefficient of a high value, for example 50 to 100. If, according to the present system, I is the sum of the magnetron current I and of a current I, supplied by an additional source, the current I depending on V and I, is given by the relation:

I= V/r(1+a) 'a/l+a I,

FIG. 2 shows I varying with 1,, V being constant.

Considering the fact that a l it is further possible to write, as previously noted in the formula (4):

According to the invention, I, must always have the same direction. By assuming that I and I, have the same sign, this implies that I is always kept lower than V/r,

Indeed V/rB is equal to the coil current I, which, when V is constant, remains substantially constant. The selection of the winding readily fulfills this condition. There are empirical laws which, for a predetermined magnetic material, determine the variations of the induction B with the coil current and the number n of winding turns. These are of the form: B h n 1 wherein h is a constant.

Considering the magnetron tube geometry, a predetermined volume is available for the winding. If d is the diameter of the wire, n is roughly proportional tO A/a. Consequently, I with other values being unchanged, is approximately proportional to d and the resistance R B varies inversely with a.

Considering again the formula (6) which, as in the prior art, gives the microwave power Pm depending on V and on the additional source voltage V,, it is known that, if the relative variations of V and V, are equal to those of the ac. primary source voltage U, this results 1n:

APm/Pm 2 AU/ U.

For each setting of Pm, and therefore of V, V, and R, it is seen by combining the formulas (5) and (6), that APm is kept at a minimum if the relative instabilities A V/ V and AV,/ V, verify the relation:

As 2IR/V, is positive, AV,/V, must be higher than AV/ V.

Therefore, according to the invention, the additional source of voltage V, is replaced by a source of higher voltage V, such that V, V,

E, is a stable voltage corresponding, for example, to the Zener diode characteristic breakdown voltage or to a gas discharge lamp stabilization voltage. Zener diodes and gas discharge lamps, as it is known, operate with a particular value of current intensity passing through them to provide very stable c.e.m. force sources having very'low and constant internal resistance.

Under such conditions, the absolute instability AV, is equal to the absolute instability AV,, but the relative instability AV,/ V, is higher than the relative instability AV,'/ V, and thus than AV/V. (AV,'/V,' AV/V) By replacing AV, in the relation (7) by AV,', V, by V, E, and AV/ V by AV,'/V,', the result is that E,, I and R verify the relation:

Within the predetermined range of values according to the assumptions made up to now, when V is constant, the microwave power Pm is proportional to I. Considering the formula (5) and taking (8) into account, it appears that the voltage V, is associated with the current I by the relation:

(9) V, =E E,/2 (1 1 wherein: 1 represents the coil current which is substantially constant when I is varied, and E represents the voltage R, 1, across the terminals of the coil having a resistance R Therefore E is also substantially constant.

If E, is assumed to be the same for any adjustment of Pm and thus of I, the relations (8) and (9) show the values to be simultaneously assigned to R and V, for each value of I. 1

Practically, the value of E, is selected within the range of values of the voltage E Many Zener diodes are available which permit a c.e.m. force E,to be obtained within a range of a few volts to tens of volts, and which are substantially stable as soon as the current passing through the diodes is over a predetermined value. It is also possible to connect several similar diodes in series by selecting a common Zener diode having a breakdown voltage of about 6 to 10 volts. Such Zener diodes are known to be particularly insensitive to temperature variations.

By way of example, the following table T1 shows the various values of R and V, for a continuous wave magnetron able to deliver, up a maximum microwave power (Pm),, from 75, 50 or 25 of (Pm all of which are regulated according to the invention.

The magnetron high voltage is 5 kV and the current intensity producing the maximum microwave power (Pm, is l A. With an efficiency of for k, and with P,,,, ,=5 kW, (F111,) is equal to 3.5 kW.

The four power levels will be obtained for currents respectively equal to l, 0.75, 0.5 and 0.25 Amperes.

A Zener diode or a series connected Zener diode assembly has been selected having a breakdown voltage E, of about 30 V and needing a minimum current I, of 0.4 A in order to consider the internal dynamic resistance as very low and constant. Under such conditions, 1,, is substantially equal to 1.4 A and, with the resistor R being 28 ohms, E is equal to 40 V.

TABLE T1 Pm w) 0.25(Pm), 0.5(Pm), 0.75(Pm)o (Pm),=3.5 kW 1 (A) 0.25 0.5 0.75 1 1, (A) 1.4 1.4 1.4 L4 EFR, l, 40 4o 40 40 R (ohm) 6O 30 2o 15 v, v 139 97 83 76 There is some freedom in selecting E The significant condition to be fulfilled is that, within the range of possible variations of I the internal dynamic resistance of the Zener diode remains very low and constant. Practically, it is possible to take into account the internal dynamic resistance of the Zener diode(s) for producing each value of R.

The configuration of the two sources of voltages V and V, is substantially simpler than those previously used in power supplies for continuous-wave magnetron. The invention is based on the fact that the two do. sources are both subject to the same relative variations as the ac. source from which they are generated. Thus, it is possible, with a large variation in the common a.c. source, to stabilize the a.c. source to a certain extent by using limited-current transformer. However, according to the present invention, the rectified voltages Vand V are not to be stabilized and only good filtering is necessary to eliminate the residual a.c. voltages.

FIG. 3 shows the circuit of an embodiment of a power supply circuit for a continuous-wave magnetron, according to the present invention. The only parts of the magnetron 1 that are shown are the cathode la and the cavity-anode lb which is directly connected to the ground. The negative high voltage applied to the cathode 1a is obtained from an ac. source U and a transformer 3 which is then rectified in a rectifierbridge 4 and filtered in a filter made of an auto-inductance and capacitors 5. The positive terminal of the rectifier-bridge 4 is connected to the ground via the coil 2 of the electromagnet.

The positive additional voltage V,,' is obtained from the same a.c. source U and a transformer 6 after having been rectified in a rectifier-bridge 7 and filtered in a filter formed by an autoinductant capacitors 8. The secondary of 6 is provided with several outputs: 6-1, 6-

2, 6-3 and 6-4, which may separately be connected to 4 the highest point of 7 via a rotating switch 9. Movement of the switch 9 permits adjustment of the voltage V,, to the proper value. The output of the filter 8 is connected, via a Zener diode 10, to four resistors 11-4, 11- 3, ll-2 and 11-1 connected in series. The opposite end of 11-1 is connected to the higher point of the coil 2. The values of the resistors 11-4 to '11-1 are respectively 15, 5,10 and 30 ohms.

A rotating switch 12 which can make contact with the taps 13-1, 13-2, 13-3 and 13-4 may put into operation, one, two, three or four of the resistors 11 from the right to the left. This provides the connections for the resistors R of the table T1. The movements of the two switches may be coupled in any suitable manner.

In a modified version of the power supply circuit shown in FIG. 3, it is possible to obtain continuous variation of the microwave power (Pm) by replacing the set of resistors 11 and the switch 12 by a rotating rheostat and also by replacing the secondary of 6 and the rotating switch 9 with a variable autotransformer. The movements of the sliding contacts of the rheostat and of the autotransformer may be coupled to provide correspondence between V,,' and R for each value of I by using the formulas (8) and (9).

It is also possible, according to the present invention, to obtain discrete levels of microwave power by varying only R, with V being constant, or V,, with R being constant. However, in both cases, a Zener diode or a set of Zener diodes will be necessary for each power level.

The case where R is kept constant will be considered first.

For each power level, I is defined and from formula (8) results in a value for E,,== 2R1.

Likewise, with formula (9), V,, is equal to:

With reference to the previous mentioned example for a magnetron delivering four microwave power levels and by selecting a resistor R of 30 ohms, the table T2 may be deduced as follows:

TABLE T2 Pm w 0.25 Pm 0.5 Pm 0.75(Pm Pm ,=3.5 kW I (A) 0.25 0.5 0.75 l I, (A) 1.4 1.4 1.4 1.4

EB (V) 40 40 40 40 Due to the fact that the selected power levels are the successive multiples of a minimum level, the values of E, are also the multiples of an initial minimum value. This is used in the circuit of FIG. 4 which slows only the circuitry of the additional source.

The differences with respect to FIG. 3 are the following: the set of four resistors is replaced by a single resistor 11, instead of a single Zener diode, there are four similar Zener diodes mounted in series (10-1 to 10-4), each one having a breakdown voltage of 15 V, and it is possible to put into operation one, two, three or four diodes from right to left by means of a rotating switch 14.

The contact position for both switches 9 and 14 of FIG. 4, at points 64 and 15-4, respectively corresponds to the maximum microwave power level.

Considering the case where V,,' is kept constant, for each microwave power level and for each value of I, E, and R are defined by using the formulas (8) and (9),

and E =m With reference to the previous mentioned example for a magnetron delivering four microwave power levels and by having V 100 V, the following Table T3 may be deduced.

The circuit of FIG. 5, which again shows only the part concerning the additional source, operates with a constant voltage V,,'. As in FIGS. 3 and 4, the transformer 6 is shown, but with a secondary having only two connections to the rectifier-bridge 7 which is also connected to the filter 8.

TABLE T3 Prn(W) 0.2S(Pm),, 0.5 Pm), 0.75(Pm)0 (Pm).,= 3.5 kW [(A) 0.25 0.5 0.75 1 IB(A) 1.4 1.4 1.4 1.4 E,,(v) 40 40 40 4o R(ohm) 36 32 2s 25 E,, v) 1s 32 42 50 The rotating switch 14 of FIG. has four positions 15-1 to 15-4 which provide a series connection of one, two, three or four Zener diodes -1 to 104, having bend voltages respectively of 18, 14, 10 and 8 volts. A rotating switch 12 with four positions 13-1 to 13-4 permits a series connection of one, two, three or four resistors 1 1-1 to 11-4 of which the values are respectively 25, 3, 4 and 4 ohms in order to obtain the resistances R of Table T3.

The position for both the switches indicated at 13-1 and -4 respectively, corresponds to the maximum microwave power level. The position 13-4 and 15-1 corresponds to the minimum power level.

The values of the resistors, Le. 25, 3, 4 and 4 ohms, are actually theoretical values. In practice, they have to be corrected to take into account the internal resistances of the Zener diodes which may be not negligible le with respect to 3 or 4 ohms. Thus the value of the resistor 11-1 will be 25 ohms minus the sum of the internal resistances of the four Zener diodes 10-1 to 10-4;

' the value of the resistor 1 1-2 will be 3 ohms plus the internal resistance of the Zener diode 10-1; that of 11-3 will be 4 ohms plus the internal resistance of 10-2; and that of 11-4 will be 4 ohms plus the internal resistance of 10-3.

In all the described examples, it has been assumed that the c.e.m. force E was provided by one or several Zener diodes. Obviously, it is possible to replace part or all of the Zener diodes by gas regulating tubes or fluorescent tubes.

While the principles of the present invention have been described in connection with several particular embodiments, it will be understood that this has been made only by way of example and does not limit the scope of the invention as set forth in the appended claims.

WHAT IS CLAIMED IS:

1. A continuous wave magnetron power supply circuit comprising:

an alternating current power source;

a magnetron tube having an enclosed cathode and anode electrodes and an external electromagnetic coil;

first direct current supply means including first rectitier and filter means connected between said alternating current power source and said magnetron electrodes and coil for supplying direct current thereto;

second direct current supply means connected between said alternating current power source and said magnetron coil for supplying additional direct current to said coil, said second direct current means including second rectifier and filter means and a plurality of series connected circuit elements including a resistor and an electron discharge device having a stable characteristic breakdown voltage; and

means for selectively adjusting said additional direct current supplied to said coil to establish a plurality of different stable power output levels.

2. The device of claim 1 wherein said plurality of series connected circuit elements includes a plurality of resistors and said selectively adjusting means includes means for connecting selected numbers of said resistors in series with said coil.

3. The device of claim 1 wherein said plurality of series connected circuit elements includes a plurality of zener diodes and said selectively adjusting means includes means for connecting selected numbers of said zener diodes in series with said coil.

4. The device of claim 1 wherein said selectively adjusting means includes means connected to said source of alternating current power for adjusting the voltage from said alternating current power source connected to said second direct current supply means.

5. The device of claim 2 wherein said selectively adjusting means includes means connected to said source of alternating current power for adjusting the voltage from said alternating current power source connected to said second direct current supply means.

6. The device of claim 2 wherein said plurality of series connected circuit elements includes a further plurality of zener diodes and said selectively adjusting means includes means for connecting selected numbers of said zener diodes in series with said resistors and coil.

7. The device of claim '3 wherein said selectively adjusting means includes means connected to said source of alternating current power for adjusting the voltage from said alternating current power source connected to said second direct current supply means. 

1. A continuous wave magnetron power supply circuit comprising: an alternating current pOwer source; a magnetron tube having an enclosed cathode and anode electrodes and an external electromagnetic coil; first direct current supply means including first rectifier and filter means connected between said alternating current power source and said magnetron electrodes and coil for supplying direct current thereto; second direct current supply means connected between said alternating current power source and said magnetron coil for supplying additional direct current to said coil, said second direct current means including second rectifier and filter means and a plurality of series connected circuit elements including a resistor and an electron discharge device having a stable characteristic breakdown voltage; and means for selectively adjusting said additional direct current supplied to said coil to establish a plurality of different stable power output levels.
 2. The device of claim 1 wherein said plurality of series connected circuit elements includes a plurality of resistors and said selectively adjusting means includes means for connecting selected numbers of said resistors in series with said coil.
 3. The device of claim 1 wherein said plurality of series connected circuit elements includes a plurality of zener diodes and said selectively adjusting means includes means for connecting selected numbers of said zener diodes in series with said coil.
 4. The device of claim 1 wherein said selectively adjusting means includes means connected to said source of alternating current power for adjusting the voltage from said alternating current power source connected to said second direct current supply means.
 5. The device of claim 2 wherein said selectively adjusting means includes means connected to said source of alternating current power for adjusting the voltage from said alternating current power source connected to said second direct current supply means.
 6. The device of claim 2 wherein said plurality of series connected circuit elements includes a further plurality of zener diodes and said selectively adjusting means includes means for connecting selected numbers of said zener diodes in series with said resistors and coil.
 7. The device of claim 3 wherein said selectively adjusting means includes means connected to said source of alternating current power for adjusting the voltage from said alternating current power source connected to said second direct current supply means. 